What is the average weight of a Border Collie?

30-40 lbs for a female (13.5-18 kg) and 40-60 lbs for a male (18-27 kg)

Though there will always be exceptions to this (i.e. larger or smaller
dogs), the large majority of Border Collies fall into this range. The
breed as a whole is a medium-sized breed. A larger Border Collie will
often be more intimidating to stubborn stock (though true power really
has little to do with overall size) but a smaller one will be faster,
enabling it to cut off fleeing stock or cover both ends of a large flock.
A fine line between the two results in the overall range of body weights
we see today.

Cattle ranchers tend to prefer the larger Border Collies as oftentimes,
pure brawn will help them move cows. The dogs tend to be a bit "stockier"
(no pun intended) and a bit broader at the shoulders and head. They are
what one might call a "thick dog", leaning more toward the body
conformation of a Rottweiler than a Whippet for instance.

Sheep ranchers, on the other hand, tend to prefer the smaller and swifter
versions, where speed is of the essence, not overall muscle weight. Some
ranchers, with bulkier sheep, may also prefer the larger and more massive
versions but in general, the medium-sized Border Collies are the ideal.